Comparing Arabidopsis receptor kinase and receptor protein-mediated immune signaling reveals BIK1-dependent differences

Pattern recognition receptors (PRRs) sense microbial patterns and activate innate immunity against attempted microbial invasions. The leucine-rich repeat receptor kinases (LRR-RK) FLS2 and EFR, and the LRR receptor protein (LRR-RP) receptors RLP23 and RLP42, respectively, represent prototypical members of these two prominent and closely related PRR families. We conducted a survey of Arabidopsis thaliana immune signaling mediated by these receptors to address the question of commonalities and differences between LRR-RK and LRR-RP signaling. Quantitative differences in timing and amplitude were observed for several early immune responses, with RP-mediated responses typically being slower and more prolonged than those mediated by RKs. Activation of RLP23, but not FLS2, induced the production of camalexin. Transcriptomic analysis revealed that RLP23-regulated genes represent only a fraction of those genes differentially expressed upon FLS2 activation. Several positive and negative regulators of FLS2-signaling play similar roles in RLP23 signaling. Intriguingly, the cytoplasmic receptor kinase BIK1, a positive regulator of RK signaling, acts as a negative regulator of RP-type immune receptors in a manner dependent on BIK1 kinase activity. Our study unveiled unexpected differences in two closely related receptor systems and reports a new negative role of BIK1 in plant immunity.

Venus flytrap carnivorous lifestyle builds on herbivore defense strategies

Although the concept of botanical carnivory has been known since Darwin's time, the molecular mechanisms that allow animal feeding remain unknown, primarily due to a complete lack of genomic information. Here, we show that the transcriptomic landscape of the Dionaea trap is dramatically shifted toward signal transduction and nutrient transport upon insect feeding, with touch hormone signaling and protein secretion prevailing. At the same time, a massive induction of general defense responses is accompanied by the repression of cell death-related genes/processes. We hypothesize that the carnivory syndrome of Dionaea evolved by exaptation of ancient defense pathways, replacing cell death with nutrient acquisition.

SLAH3-type anion channel expressed in poplar secretory epithelia operates in calcium kinase CPK-autonomous manner

Extrafloral nectaries secrete a sweet sugar cocktail that lures predator insects for protection from foraging herbivores. Apart from sugars and amino acids, the nectar contains the anions chloride and nitrate. Recent studies with Populus have identified a type of nectary covered by apical bipolar epidermal cells, reminiscent of the secretory brush border epithelium in animals. Border epithelia operate transepithelial anion transport, which is required for membrane potential and/or osmotic adjustment of the secretory cells. In search of anion transporters expressed in extrafloral nectaries, we identified PttSLAH3 (Populus tremula × Populus tremuloides SLAC1 Homologue3), an anion channel of the SLAC/SLAH family. When expressed in Xenopus oocytes, PttSLAH3 displayed the features of a voltage-dependent anion channel, permeable to both nitrate and chloride. In contrast to the Arabidopsis SLAC/SLAH family members, the poplar isoform PttSLAH3 is independent of phosphorylation activation by protein kinases. To understand the basis for the autonomous activity of the poplar SLAH3, we generated and expressed chimera between kinase-independent PttSLAH3 and kinase-dependent Arabidopsis AtSLAH3. We identified the N-terminal tail and, to a lesser extent, the C-terminal tail as responsible for PttSLAH3 kinase-(in)dependent action. This feature of PttSLAH3 may provide the secretory cell with a channel probably controlling long-term nectar secretion.

Venus Flytrap HKT1-Type Channel Provides for Prey Sodium Uptake into Carnivorous Plant Without Conflicting with Electrical Excitability

The animal diet of the carnivorous Venus flytrap, Dionaea muscipula, contains a sodium load that enters the capture organ via an HKT1-type sodium channel, expressed in special epithelia cells on the inner trap lobe surface. DmHKT1 expression and sodium uptake activity is induced upon prey contact. Here, we analyzed the HKT1 properties required for prey sodium osmolyte management of carnivorous Dionaea. Analyses were based on homology modeling, generation of model-derived point mutants, and their functional testing in Xenopus oocytes. We showed that the wild-type HKT1 and its Na(+)- and K(+)-permeable mutants function as ion channels rather than K(+) transporters driven by proton or sodium gradients. These structural and biophysical features of a high-capacity, Na(+)-selective ion channel enable Dionaea glands to manage prey-derived sodium loads without confounding the action potential-based information management of the flytrap.

The Venus Flytrap Dionaea muscipula Counts Prey-Induced Action Potentials to Induce Sodium Uptake

Carnivorous plants, such as the Venus flytrap (Dionaea muscipula), depend on an animal diet when grown in nutrient-poor soils. When an insect visits the trap and tilts the mechanosensors on the inner surface, action potentials (APs) are fired. After a moving object elicits two APs, the trap snaps shut, encaging the victim. Panicking preys repeatedly touch the trigger hairs over the subsequent hours, leading to a hermetically closed trap, which via the gland-based endocrine system is flooded by a prey-decomposing acidic enzyme cocktail. Here, we asked the question as to how many times trigger hairs have to be stimulated (e.g., now many APs are required) for the flytrap to recognize an encaged object as potential food, thus making it worthwhile activating the glands. By applying a series of trigger-hair stimulations, we found that the touch hormone jasmonic acid (JA) signaling pathway is activated after the second stimulus, while more than three APs are required to trigger an expression of genes encoding prey-degrading hydrolases, and that this expression is proportional to the number of mechanical stimulations. A decomposing animal contains a sodium load, and we have found that these sodium ions enter the capture organ via glands. We identified a flytrap sodium channel DmHKT1 as responsible for this sodium acquisition, with the number of transcripts expressed being dependent on the number of mechano-electric stimulations. Hence, the number of APs a victim triggers while trying to break out of the trap identifies the moving prey as a struggling Na⁺-rich animal and nutrition for the plant.

Video Abstract

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Carnivorous Dionaea muscipula captures and processes nutrient- and sodium-rich prey

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Via mechano-sensor stimulation, an animal meal is recognized, captured, and processed

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Mechano-electrical waves induce JA signaling pathways that trigger prey digestion

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Number of stimulations controls the production of digesting enzymes and uptake modules

An RLP23-SOBIR1-BAK1 complex mediates NLP-triggered immunity

Plants and animals employ innate immune systems to cope with microbial infection. Pattern-triggered immunity relies on the recognition of microbe-derived patterns by pattern recognition receptors (PRRs). Necrosis and ethylene-inducing peptide 1-like proteins (NLPs) constitute plant immunogenic patterns that are unique, as these proteins are produced by multiple prokaryotic (bacterial) and eukaryotic (fungal, oomycete) species. Here we show that the leucine-rich repeat receptor protein (LRR-RP) RLP23 binds in vivo to a conserved 20-amino-acid fragment found in most NLPs (nlp20), thereby mediating immune activation in Arabidopsis thaliana. RLP23 forms a constitutive, ligand-independent complex with the LRR receptor kinase (LRR-RK) SOBIR1 (Suppressor of Brassinosteroid insensitive 1 (BRI1)-associated kinase (BAK1)-interacting receptor kinase 1), and recruits a second LRR-RK, BAK1, into a tripartite complex upon ligand binding. Stable, ectopic expression of RLP23 in potato (Solanum tuberosum) confers nlp20 pattern recognition and enhanced immunity to destructive oomycete and fungal plant pathogens, such as Phytophthora infestans and Sclerotinia sclerotiorum. PRRs that recognize widespread microbial patterns might be particularly suited for engineering immunity in crop plants.

Integration of trap- and root-derived nitrogen nutrition of carnivorous Dionaea muscipula

Carnivorous Dionaea muscipula operates active snap traps for nutrient acquisition from prey; so what is the role of D. muscipula's reduced root system? We studied the capacity for nitrogen (N) acquisition via traps, and its effect on plant allometry; the capacity of roots to absorb NO₃(-), NH₄(+) and glutamine from the soil solution; and the fate and interaction of foliar- and root-acquired N. Feeding D. muscipula snap traps with insects had little effect on the root : shoot ratio, but promoted petiole relative to trap growth. Large amounts of NH₄(+) and glutamine were absorbed upon root feeding. The high capacity for root N uptake was maintained upon feeding traps with glutamine. High root acquisition of NH₄(+) was mediated by 2.5-fold higher expression of the NH₄(+) transporter DmAMT1 in the roots compared with the traps. Electrophysiological studies confirmed a high constitutive capacity for NH₄(+) uptake by roots. Glutamine feeding of traps inhibited the influx of (15)N from root-absorbed (15)N/(13)C-glutamine into these traps, but not that of (13)C. Apparently, fed traps turned into carbon sinks that even acquired organic carbon from roots. N acquisition at the whole-plant level is fundamentally different in D. muscipula compared with noncarnivorous species, where foliar N influx down-regulates N uptake by roots.

Effects of hepatic protein tyrosine phosphatase 1B and methionine restriction on hepatic and whole-body glucose and lipid metabolism in mice

AIMS

Methionine restriction (MR) and hepatic protein tyrosine phosphatase 1B (PTP1B) knockdown both improve hepatic insulin sensitivity by targeting different proteins within the insulin signaling pathway, as well as diminishing hepatic triglyceride content through decreasing hepatic lipogenesis. We hypothesized that a combined approach of hepatic PTP1B inhibition and methionine restriction could lead to a synergistic effect on improvements in glucose homeostasis and lipid metabolism.

METHODS

Male and female hepatic PTP1B knockout (Alb-Ptp1b(-/-)) and control wild-type (Ptp1b(fl/fl)) mice were maintained on control diet (0.86% methionine) or MR diet (0.172% methionine) for 8weeks. Body weight and food intake were recorded and physiological tests for whole-body glucose homeostasis were performed. Serum and tissues were analyzed biochemically.

RESULTS

MR decreased body weight and increased food intake in Ptp1b(fl/fl) mice as expected, without changing PTP1B protein expression levels or activity. In females, MR treatment alone improved glucose tolerance in Ptp1b(fl/fl) mice, which was further amplified with hepatic PTP1B deficiency. However, other markers of glucose homeostasis were similar between MR-fed groups. In males, MR improved glucose homeostasis in both, Alb-Ptp1b(-/-) and wild-type Ptp1b(fl/fl) mice to a similar extent. Hepatic PTP1B inhibition in combination with MR could not further enhance insulin-stimulated hepatic protein kinase B/Akt phosphorylation compared to MR treatment alone and therefore led to no further increase in hepatic insulin signaling. The combined treatment did not further improve lipid metabolism relative to MR diet alone.

CONCLUSIONS

Methionine restriction improves glucose and lipid homeostasis; however, adding hepatic PTP1B inhibition to MR is unlikely to yield any additional protective effects.

The Dionaea muscipula ammonium channel DmAMT1 provides NH₄⁺ uptake associated with Venus flytrap's prey digestion

BACKGROUND

Ammonium transporter (AMT/MEP/Rh) superfamily members mediate ammonium uptake and retrieval. This pivotal transport system is conserved among all living organisms. For plants, nitrogen represents a macronutrient available in the soil as ammonium, nitrate, and organic nitrogen compounds. Plants living on extremely nutrient-poor soils have developed a number of adaptation mechanisms, including a carnivorous lifestyle. This study addresses the molecular nature, function, and regulation of prey-derived ammonium uptake in the Venus flytrap, Dionaea muscipula, one of the fastest active carnivores.

RESULTS

The Dionaea muscipula ammonium transporter DmAMT1 was localized in gland complexes where its expression was upregulated upon secretion. These clusters of cells decorating the inner trap surface are engaged in (1) secretion of an acidic digestive enzyme cocktail and (2) uptake of prey-derived nutrients. Voltage clamp of Xenopus oocytes expressing DmAMT1 and membrane potential recordings with DmAMT1-expressing Dionaea glands were used to monitor and compare electrophysiological properties of DmAMT1 in vitro and in planta. DmAMT1 exhibited the hallmark biophysical properties of a NH4(+)-selective channel. At depolarized membrane potentials (Vm = 0), the Km (3.2 ± 0.3 mM) indicated a low affinity of DmAMT1 for ammonium that increased systematically with negative going voltages. Upon hyperpolarization to, e.g., -200 mV, a Km of 0.14 ± 0.015 mM documents the voltage-dependent shift of DmAMT1 into a NH4(+) transport system of high affinity.

CONCLUSIONS

We suggest that regulation of glandular DmAMT1 and membrane potential readjustments of the endocrine cells provide for effective adaptation to varying, prey-derived ammonium sources.

Cadmium and selenium modulate slow vacuolar channels in rape (Brassica napus) vacuoles

Currents flowing through slow vacuolar SV channels of rape (Brassica napus) growing on media supplemented with Cd²+ (400 μM), and/or SeO₄(²⁻) (2μM) were examined. The aim of the study was to investigate the role of Cd²+ in modulation of SV channel activity and to determine whether Se reverses the effect of cadmium. Vacuoles were isolated using a quick surgical method to avoid application of any cell wall-degrading enzymes. Vacuoles of rape exhibited typical SV channel activity with slow activation at positive potentials and strong rectification into the vacuolar lumen. Single-channel conductance in cytoplasm-side-out tonoplast patches ranged between 68.8±1.9 pS in the control, 80.1±2.5pS, in Cd²+, 74.2±2.4 pS in Cd²+/selenate, and 80.1±1.8 pS in selenate-pretreated plants. The lack of a clear tendency was likely an effect of equilibration of the pipette solution (without Cd²+/SeO₄(²⁻) with that of the luminal side of the vesicles. In the vacuole-attached configuration, in which natural vacuolar solution was not exchanged, there was a significant reduction in single-channel conductance in the Cd²+ (40.3±2.8 pS), Cd²+/selenate (47.1±2.8 pS) and selenate-pretreated (42.3±1.4 pS) plants, compared to the control (60.2±1.7 pS). The reduction in single-channel conductance only partially explained the significant decline in the densities of ion current flowing through the vacuolar membrane in the whole-vacuole configuration in the plants growing on Cd²+ and Cd²+/selenate media. Thus, Cd²+ accumulation in the vacuole reduced the number of active SV channels from 0.28±0.05 μm⁻² in the control to 0.021±0.005 and 0.039±0.004 μm⁻² in Cd²+ and Cd²+/selenate-pretreated plants, respectively.

Early signaling through the Arabidopsis pattern recognition receptors FLS2 and EFR involves Ca-associated opening of plasma membrane anion channels

The perception of microbes by plants involves highly conserved molecular signatures that are absent from the host and that are collectively referred to as microbe-associated molecular patterns (MAMPs). The Arabidopsis pattern recognition receptors FLAGELLIN-SENSING 2 (FLS2) and EF-Tu receptor (EFR) represent genetically well studied paradigms that mediate defense against bacterial pathogens. Stimulation of these receptors through their cognate ligands, bacterial flagellin or bacterial elongation factor Tu, leads to a defense response and ultimately to increased resistance. However, little is known about the early signaling pathway of these receptors. Here, we characterize this early response in situ, using an electrophysiological approach. In line with a release of negatively charged molecules, voltage recordings of microelectrode-impaled mesophyll cells and root hairs of Col-0 Arabidopsis plants revealed rapid, dose-dependent membrane potential depolarizations in response to either flg22 or elf18. Using ion-selective microelectrodes, pronounced anion currents were recorded upon application of flg22 and elf18, indicating that the signaling cascades initiated by each of the two receptors converge on the same plasma membrane ion channels. Combined calcium imaging and electrophysiological measurements revealed that the depolarization was superimposed by an increase in cytosolic calcium that was indispensable for depolarization. NADPH oxidase mutants were still depolarized upon elicitor stimulation, suggesting a reactive oxygen species-independent membrane potential response. Furthermore, electrical signaling in response to either flg22 or elf 18 critically depends on the activity of the FLS2-associated receptor-like kinase BAK1, suggesting that activation of FLS2 and EFR lead to BAK1-dependent, calcium-associated plasma membrane anion channel opening as an initial step in the pathogen defense pathway.

Perception of the Arabidopsis danger signal peptide 1 involves the pattern recognition receptor AtPEPR1 and its close homologue AtPEPR2

Plasma membrane-borne pattern recognition receptors, which recognize microbe-associated molecular patterns and endogenous damage-associated molecular patterns, provide the first line of defense in innate immunity. In plants, leucine-rich repeat receptor kinases fulfill this role, as exemplified by FLS2 and EFR, the receptors for the microbe-associated molecular patterns flagellin and elongation factor Tu. Here we examined the perception of the damage-associated molecular pattern peptide 1 (AtPep1), an endogenous peptide of Arabidopsis identified earlier and shown to be perceived by the leucine-rich repeat protein kinase PEPR1. Using seedling growth inhibition, elicitation of an oxidative burst and induction of ethylene biosynthesis, we show that wild type plants and the pepr1 and pepr2 mutants, affected in PEPR1 and in its homologue PEPR2, are sensitive to AtPep1, but that the double mutant pepr1/pepr2 is completely insensitive. As a central body of our study, we provide electrophysiological evidence that at the level of the plasma membrane, AtPep1 triggers a receptor-dependent transient depolarization through activation of plasma membrane anion channels, and that this effect is absent in the double mutant pepr1/pepr2. The double mutant also fails to respond to AtPep2 and AtPep3, two distant homologues of AtPep1 on the basis of homology screening, implying that the PEPR1 and PEPR2 are responsible for their perception too. Our findings provide a basic framework to study the biological role of AtPep1-related danger signals and their cognate receptors.

Slow vacuolar channels in vacuoles from winter and spring varieties of rape (Brassica napus)

Currents passing through slowly activating vacuolar channels (SV) in isolated vacuoles from winter (Górczański) and spring (Młochowski) varieties of rape (Brassica napus) were examined using the patch-clamp technique. Eight-week-long vernalization at 5/2 degrees C (day/night) was applied to obtain the generative stage of winter rape. SV channels of vacuoles isolated from vegetative (rosette) and generative leaves of both varieties were examined in order to investigate a possible role of these ion channels in rape flowering. Single SV channel conductance measured in a vacuole-attached configuration (natural cell sap) ranged from 60 to 83 pS. Lower values were observed in the generative leaves of both varieties. Unitary conductance measured in excised cytoplasm-out membrane patches did not differ significantly among the experimental variants, with the exception of spring generative vacuoles, where it was significantly lower. There was also no difference in SV current densities measured in the whole-vacuole configuration. Gibberellic acid (GA(3)) (2mg/l) caused lowering of macroscopic SV currents by 20%, and had no significant effect on the single channel conductance. We conclude that SV channels play a role in rape vernalization and flowering owing to their multifactor regulation abilities rather than structural changes.

Complex relationship between growth and circumnutations in Helianthus annuus stem

The growth and circumnutation of the stem of three-week old Helianthus annuus in the 16:8 h light:dark photoperiod were monitored using an angular position-sensing transducer and a time lapse photography system. It was found that the rate of growth and circumnutation reached a high level in the dark stage; in the light stage, however, only the growth rate reached the same high level, whereas the circumnutations were weak. These results showed that in the light stage the stem circumnutation was downregulated more strongly than the growth. Short-term stem responses to darkening and illumination were a further display of the relation between growth and circumnutations. Switching off the light caused an increase in the growth and circumnutation rate. In some cases it was accompanied by changes in the rotation direction. On the other hand, switching the light on caused an immediate transient (several-minute long) decrease in the growth rate resulting in stem contraction, and this was accompanied by an almost complete pause of circumnutation. Additionally, under light, there occurred a subsequent decrease in the magnitude, disturbance of circumnutation trajectory and, in some cases, changes in the direction of rotation. The observed stem contraction and disturbance of circumnutation imply the occurrence of turgor changes in sunflower stem, which may be caused by a non-wounding, darkening or illumination stimulus. Our experiments indicate that the disturbances of the growth rate are accompanied by changes in circumnutation parameters but we have also seen that there is no simple quantitative relation between growth rate and circumnutation rate.

Light- and dark-induced action potentials in Physcomitrella patens

Glass microelectrodes were inserted into Physcomitrella patens gametophyte leaves and action potentials (APs) were recorded in response to sudden illumination as well as after darkening, i.e., when the dark-induced membrane depolarization crossed a threshold. Application of 5 mM La(3+) (a calcium channel inhibitor), 10 mM TEA(+) (a potassium channel inhibitor) and increased free Ca(2+) resulted in a loss of excitability. Lack of Ca(2+) in the external medium did not prevent APs from occurring. It was concluded that during light- dark-induced excitation of Physcomitrella patens, APs might rely upon calcium influxes from the intracellular compartments. APs were not blocked by the proton pump inhibitors (DES, DCCD), although the resting potential (RP) diminished significantly.

Cold transiently activates calcium-permeable channels in Arabidopsis mesophyll cells

Living organisms are capable of discriminating thermal stimuli from noxious cold to noxious heat. For more than 30 years, it has been known that plant cells respond to cold with a large and transient depolarization. Recently, using transgenic Arabidopsis (Arabidopsis thaliana) expressing the calcium-sensitive protein aequorin, an increase in cytosolic calcium following cold treatment was observed. Applying the patch-clamp technique to Arabidopsis mesophyll protoplasts, we could identify a transient plasma membrane conductance induced by rapid cooling. This cold-induced transient conductance was characterized as an outward rectifying 33 pS nonselective cation channel. The permeability ratio between calcium and cesium was 0.7, pointing to a permeation pore >3.34 A (ø of cesium). Our experiments thus provide direct evidence for the predicted but not yet measured cold-activated calcium-permeable channel in plants.

The Functional Significance of Individual Variation in Basal Metabolic Rate

Basal metabolic rate (BMR) was established as a common reference point allowing comparable measures across different individuals and species. BMR is often regarded as a minimal rate of metabolism compatible with basic processes necessary to sustain life. One confusing aspect, however, is that BMR is highly variable, both within and between species. A potential explanation for this variability is that while individuals with high BMRs may suffer the disadvantage of having to feed for longer to cover the extra energy demands, this may be offset by advantages that accrue because of the high metabolic rate. One suggested advantage is that high levels of BMR are a consequence of maintaining a morphology that permits high rates of the maximal sustained rate of metabolism (SusMR)--the rate of metabolism that can be sustained for days or weeks. We have been studying the energetics of MF1 laboratory mice during peak lactation to investigate this idea. In this article, we review some of our work in connection with three particular predictions that derive from the hypothesised links among morphology, basal metabolism, and sustained metabolic rate. By comparing groups of individuals, for example, lactating and nonlactating individuals, the patterns that emerge are broadly consistent with the hypothesis that BMR and SusMR are linked by morphology. Lactating mice have bigger organs connected with energy acquisition and utilisation, greater resting metabolic rates in the thermoneutral zone, called RMRt (approximately equivalent to BMR), and high sustainable rates of maximal energy intake. However, when attempts are made to establish these relationships across individuals within lactating mice, the associations that are anticipated are either absent or very weak and depend on shared variation due to body mass. At this level there is very little support for the suggestion that variation in RMRt (and thus BMR) is sustained by associations with SusMR.

Global transcriptional analysis of the phosphate starvation response in Sinorhizobium meliloti strains 1021 and 2011

The global response to phosphate starvation was analysed at the transcriptional level in two closely related strains of Sinorhizobium meliloti, Rm1021 and Rm2011. The Pho regulon is known to be induced by PhoB under conditions of phosphate limitation. Ninety-eight genes were found to be significantly induced (more than three-fold) in a phoB -dependent manner in phosphate-stressed cells, and phoB -independent repression of 86 genes was observed. Possible roles of these genes in the phosphate stress response are discussed. Twenty new putative PHO box sequences were identified in regions upstream of 17 of the transcriptional units that showed phoB -dependent, or partially phoB -dependent, regulation, indicating direct regulation of these genes by PhoB. Despite the overall similarity between the phosphate stress responses in Rm1021 and Rm2011, lower induction rates were found for a set of phoB -dependent genes in Rm1021. Moreover, Rm1021 exhibited moderate constitutive activation of 12 phosphate starvation-inducible, phoB -dependent genes when cells were grown in a complex medium. A 1-bp deletion was observed in the pstC ORF in Rm1021, which results in truncation of the protein product. This mutation is probably responsible for the expression of phosphate starvation-inducible genes in Rm1021 in the absence of phosphate stress.

Low-temperature-induced transmembrane potential changes in mesophyll cells of Arabidopsis thaliana, Helianthus annuus and Vicia faba

Glass microelectrodes were inserted into mesophyll cells of intact leaves from higher plants: Arabidopsis thaliana, Helianthus annuus and Vicia faba var minor, and transient membrane potential changes were recorded in response to a sudden temperature drop. The cold-induced potential changes were unaffected by an anion channel inhibitor (anthracene-9-carboxylic acid) and potassium channel inhibitor (tetraethyl ammonium ion). Verapamil, a calcium channel inhibitor, caused significant suppression of the cold-induced potential changes. In the presence of calmoduline antagonists (trifluoperazine and N-6-aminohexyl-5-chloro-1-naphtalenesulphonamide), their amplitudes decreased and their durations were prolonged. Neomycin, which suppresses phospholipase C, also caused substantial inhibition of the amplitudes of the cold-induced potential changes. It is concluded that cold-evoked membrane potential changes are due to calcium influxes from both the apoplast and internal stores.

Low-temperature induced transmembrane potential changes in the liverwort Conocephalum conicum

Intracellular microelectrode measurements revealed that the liverwort Conocephalum conicum generates all-or-none action potentials (APs) in response to a sudden temperature drop. In plants with anion and potassium conductance blocked, dose-dependent voltage transients (VTs) were evoked by cold stimuli. These VTs did not propagate. When the external concentration of Ca(2+) was decreased or calcium channel inhibitors (La(3+), Gd(3+), verapamil, Mg(2+), Mn(2+)) were used, inhibition of VTs was observed. Amplitudes of both APs and VTs grew when Sr(2+) ions, known to release calcium from internal stores, were added to the medium. Neomycin, which suppresses phospholipase C and indirectly affects inositol triphosphate formation, caused substantial inhibition of both APs and VTs. It is concluded that a temperature drop elucidated membrane potential changes due to calcium influx both from external and internal stores.

Expression pattern of Rhizobium etli ccmIEFH genes involved in c-type cytochrome maturation

In different bacterial species, ccmIEFH genes have been suggested to code for subunits of a bacterial haem-lyase catalyzing the covalent attachment of haem to c-type apoproteins. In Rhizobium etli CE3 there are two copies of ccmIEFH: one in the chromosome and the other located in plasmid pf. However, the null phenotype of chromosomal ccmF mutant indicates that the gene locus of plasmid pf is not functional. Two ccmI chromosomal mutants, previously isolated, produced detectable levels of c-type cytochromes under certain culture conditions in contrast with the ccmF mutant, suggesting that ccmF could be transcribed independently. The transcriptional organization of ccmIEFH operon was established. Two promoters from the chromosomal locus were mapped by primer extension, one located upstream of ccmI and the second located upstream of ccmF. The regulation of the expression of both promoters was studied using appropriate lacZ gene fusions (ccmI-lacZ and ccmEF-lacZ). The ccmI-lacZ gene fusion was expressed in complex medium, during exponential growth, under microaerobic conditions and in a R. etli mutant that accumulates reducing power, conditions where a higher respiration rate could be limited by c-type cytochrome content. The ccmEF-lacZ fusion was also primarily expressed in complex medium and under microaerophilic conditions. The finding of two independent promoters in this gene locus could suggest that the step catalyzed by CcmFH could be a rate-limiting step for c-type cytochrome assembly under certain culture conditions.

Effect of amlodipine therapy on the monoclonal antibody 3G8-induced calcium signal in polymorphonuclear leukocytes of hemodialysis patients

Chronic renal failure is associated with impaired phagocytosis. This was attributed to the PTH-induced elevation of basal levels of [Ca2+]i of polymorphonuclear leukocytes (PMNLs) and to the small calcium transient induced by the ligation of the Fcgamma RIII receptors of these cells with 3G8 monoclonal antibodies. The blocking of the action of PTH on the PMNLs of patients with chronic renal failure by their treatment with a calcium channel blocker normalized the basal levels of [Ca2+]i of the PMNLs and reversed the defect in their phagocytic property. It is not known whether such therapy would also restore the calcium transient in the PMNLs in response to 3G8 monoclonal antibody to normal. We examined this issue in 12 normal subjects and 18 hemodialysis patients; 9 of them were treated with the calcium channel blocker, amlodipine, and the other 9 did not receive such therapy. The treatment with amlodipine normalized [Ca2+]i of PMNLs as well as the calcium transient in response to 3G8 monoclonal antibody and reversed the defect in their phagocytosis. It is concluded that chronic renal failure is associated with deranged calcium homeostasis of PMNLs which causes abnormalities in the function of Fcgamma RIII receptors and consequently results in impaired phagocytosis. Therapy with a calcium channel blocker can reverse all these derangements in metabolism and function of PMNLs.

Topologic preconditions for ring chromatids

The paper discusses a mechanism apt to convert the topology of a straight chromatid into one that could lead to the formation of a O-ring. It is predicted that such conversions might occur only in exceptional circumstances, due to a combination of poorly correlated events.

Mechanisms through which high glucose concentration raises [Ca2+]i in renal proximal tubular cells

BACKGROUND

The basal levels of cytosolic calcium ([Ca2+]i) of renal proximal tubular cells of rats with streptozotocin-induced diabetes are elevated. It is possible that this phenomenon is mediated by the hyperglycemia, which may cause both increased calcium influx into and/or decreased calcium efflux out of these cells.

METHODS

We examined whether high glucose concentration in vitro causes acute rise in [Ca2+]i of freshly isolated renal proximal tubular cells and explored the pathways that are involved in such an event.

RESULTS

There were dose and time dependent increments in [Ca2+]i of renal proximal tubular cells exposed to high concentrations of glucose. A similar effect was observed with equimolar concentrations of mannitol or choline chloride but not urea. A substantial part of the rise in [Ca2+]i was inhibited when the media contained verapamil, nifedipine, amlodipine or ryanodine and when the cells were placed in a calcium free media. Inhibitors of G protein(s) (GDPbetaS or pertussis toxin), inhibitors of cAMP-protein kinase A pathway (RpcAMP or H-89), inhibitors of protein kinase C (staurosporine or calphostin) and inhibitor of Na+-H+ exchanger (HOE 694) blocked the rise in a dose dependent manner. High glucose concentration also caused a decrease in ATP content of these cells and a reduction in the Vmax of their Ca2+ATPase.

CONCLUSIONS

The results are consistent with the formulation that the osmotic activity (cell shrinkage) of the high glucose concentration may activate a stretch receptor with subsequent stimulation of various cellular pathways including G protein(s), cAMP-protein kinase A and phospholipase C systems and calcium channels. Activation of these cellular pathways permits both calcium influx into renal tubular cells and mobilization of calcium from their intracellular stores. Further, a decrease in calcium efflux secondary to the reduction in the Vmax of Ca2+ ATPase may occur. It is possible that the rise in [Ca2+]i is critical for the stimulation of the events that lead to restoration of cell volume to normal.

Developmental regulation of the gibberellin biosynthetic gene GA1 in Arabidopsis thaliana

The GA1 gene of Arabidopsis thaliana encodes ent-kaurene synthase A (KSA), which catalyzes the first committed step in the biosynthetic pathway of the plant hormone gibberellin (GA). Its location in the GA biosynthetic pathway has led to speculation that KSA regulation is one of the controlling steps. However, because KSA activity is so low that it is only measurable in Arabidopsis siliques, GA1 promoter-GUS reporter gene fusions and quantitative reverse transcriptase-polymerase chain reaction (RT-PCR) were used to examine the expression pattern of GA1. The results from this study indicate that GA1 gene expression is highly regulated during growth and development, and it is restricted to specific cell types at the sites of expression. GA1 promoter activity is highest in rapidly growing tissues, e.g. shoot apicas, root tips, developing flowers and seeds. It is also active in the vascular tissue of some non-growing organs, such as expanded leaves, suggesting that these leaves may be a site of GA synthesis for transport to other organs. It was also found that the first one or two introns in the GA1 gene are required for proper expression. Because of the high degree of regulation, GA1 may act as a gatekeeper, controlling the flow of metabolites into the GA biosynthetic pathway, while the levels of specific bioactive GAs are controlled by other downstream steps.

Bioreductive activation of catechol estrogen-ortho-quinones: aromatization of the B ring in 4-hydroxyequilenin markedly alters quinoid formation and reactivity

There is a clear association between excessive exposure to estrogens and the development of cancer in several tissues including breast and endometrium. The risk factors for women developing these cancers are all associated with longer estrogen exposure, as may be facilitated by early menses, late menopause and long-term estrogen replacement therapy. Equilenin (1,3,5(10),6,8-estrapentaen-3-ol-17-one) or its 17-hydroxylated analogs make up 15% of the most widely prescribed estrogen replacement formulation, Premarin, and yet there is very little information on the human metabolism of these estrogens. In this study, we synthesized the catechol metabolite of equilenin, 4-hydroxyequilenin, and examined how aromatization of the B ring affects the formation and reactivity of the o-quinone (3,5-cyclohexadien-1,2-dione). 4-Hydroxyequilenin-o-quinone is much more redox-active and longer-lived than the endogenous catechol estrone-o-quinones, which suggests that the mechanism(s) of toxicity of the former could be quite different. Interestingly, the rate of reduction of the 4-hydroxyequilenin-o-quinone is increased at least 13-fold in the presence of NAD(P)H:quinone oxidoreductase (DT-diaphorase). Once NADH is consumed however, the catechol auto-oxidized rapidly to the o-quinone. NADH consumption was accompanied by dicumarol-sensitive oxygen uptake both with the purified enzyme and with cytosol from human melanoma cells with high levels of DT-diaphorase activity. P450 reductase and rat liver microsomes also catalyzed NADPH consumption and oxygen uptake. 4-Hydroxyestrone-o-quinone was also rapidly reduced by NAD(P)H; however, this o-quinone does not auto-oxidize and once the o-quinone is reduced the reaction terminates. Including oxidative enzymes in the incubation completes the redox couple and 4-hydroxyestrone-o-quinone behaves like 4-hydroxyequilenin-o-quinone. These data suggest that reduction of estrogen-o-quinones may not result in detoxification. Instead this could represent a cytotoxic mechanism involving consumption of reducing equivalents (NADH/NADPH) as well as formation of superoxide and other reactive oxygen species leading to oxidative stress. Finally, we have compared the cytotoxicity of 4-hydroxyequilenin with that of the estrone catechols in human melanoma cells. 4-Hydroxyequilenin is 5-fold more toxic in these cells compared with 4-hydroxyestrone (ED50 = 7.8 versus 38 microM, respectively) suggesting that formation of the longer-lived redox-active 4-hydroxyequilenin-o-quinone was responsible for the cytotoxic differences. These results substantiate the conclusion that the involvement of quinoids in catechol estrogen toxicity depends on a combination of the rate of formation of the o-quinone, the lifetime of the o-quinone, and the electrophilic/redox reactivity of the quinoids.